Creepers never used to damage the player if the player was in water when the creeper exploded... but I was watching a vanilla minecraft v1.14.4 stream today and the player was blown up by a creeper when they were both standing in the water.... when did they change that?
As far as I remember, creepers always damaged players, even in water. What creepers don't damage when in water (and still don't) was blocks in the world. So water prevents creeper holes. But Creepers still damaged entities, like players or items floating on the ground/water.